Three new cases of New World screwworm, a parasitic fly that painfully infects animals and poses a threat to humans, were confirmed by US officials on June 9, bringing the total number of domestic cases from the previous week to five.
Experts estimate that a widespread outbreak could devastate wildlife and cause $1.8 billion in economic damage to Texas, including potentially higher beef prices due to infected cattle.
Beef prices have risen steadily since December 2020 due to strong demand and the smallest U.S. cattle herd in 75 years, the American Farm Bureau Federation reports.
New World screwworm flies, also known as NWS, are the size of common houseflies. They are slated to advance throughout Mexico and Central America through 2023, according to the Centers for Disease Control and Prevention. The first US case was found in Texas on June 3.
Screwworm flies cause myiasis, an infection of flesh-eating worms that burrow into the skin of living animals, causing severe and often fatal damage. According to the Animal and Plant Health Inspection Service, the fly is named for how its insects “bite or pierce open wounds using their sharp mouthparts.”
New World screwworm begins resurgence in Central America
The Canadian Food Inspection Agency announced on June 5 that it was placing restrictions on Texas livestock due to the screwworm outbreak.
Texas Gov. Greg Abbott declared a state of disaster on June 5, citing the “imminent threat” posed by screwworm. Texas officials said Canadian authorities were overreacting.
The U.S. Department of Agriculture says the parasite only infects live animals, wildlife, birds, pets, and in some rare cases, humans. It does not infect meat or fruits.
Where was screwworm discovered?
The five confirmed cases are as follows:
- Zavala County, Texas: two calves.
- La Salle County, Texas: one calf.
- Gillespie County, Texas: A goat.
- Lee County, New Mexico: A dog.
Screwworm is native to South America and the Caribbean. According to the CDC, it was eradicated in the United States in the 1960s, in Mexico in the 1970s, and in much of Central America in the early 2000s.
But the CDC said new cases were reported in Panama, Costa Rica, Nicaragua and Honduras in October 2024. This led to concerns that the insects would spread north to the United States.
The USDA said the first case in southern Mexico was reported in the United States in November.
Although the United States and Mexico are working together to eliminate NWS in Mexico, the USDA said, “There has been unacceptable northward progress of NWS, and additional actions must be taken to slow the northern progress of this deadly parasitic fly.”
What is so dangerous about NWS?
According to the CDC, NWS flies lay eggs “on open wounds or other exposed body parts of living, warm-blooded animals.” Even a wound as small as a tick bite can attract flies.
The eggs hatch and become larvae or maggots. The CDC says they enter wounds to feed on live flesh. This is unlike most other types of insects, which feed on rotting flesh.
After feeding, the larvae fall to the ground and burrow into it. They later emerge as adult screwworm flies, the CDC says.
As you might expect, digging worms into living creatures is extremely painful.
What is being done to combat screwworm?
Some of the Department of Agriculture’s countermeasures include:
- Metapa, Mexico: USDA invested $21 million to modernize an existing fruit fly facility in Mexico. It is expected to open this summer and produce 60 million to 100 million sterile NWS flies a week.
- Edinburg, Texas: A $750 million sterile fly production facility is under construction at Moore Air Base in South Texas. It will have an estimated capacity of 300 million sterile flies per week and is expected to open in November 2027.
- Pacora, Panama: USDA is moving 100 million sterile flies a week from the COPEG facility in Panama. They spread within and just north of the affected areas in Mexico.
What are the symptoms of NWS infection in humans?
The USDA says NWS infestations are very painful. If you have an infection, you may notice worms around or in an open wound or in your nose, eyes, or mouth. Symptoms may include:
- Unexplained skin lesions (sores or wounds) that do not heal.
- Lesions or sores on the skin that get worse over time.
- Painful skin sores or wounds.
- Bleeding from open wounds.
- Feeling the movement of larvae inside skin wounds or sores, nose, mouth or eyes.
- Seeing insects around or in open wounds.
- Foul odor from the site of infection.
Secondary bacterial infection may occur and cause fever or chills.
In animals, symptoms include:
- Irritable behaviour.
- Head shaking.
- The smell of decay.
- Evidence of fly strike.
- Presence of fly insects in wounds.
What should you do if you become infected?
The CDC says any worms present in a wound or body part should be removed by a medical professional. Do not attempt to remove them yourself.
Contributing: Greta Cross, John Bacon, Mike Snyder and Eduardo Cuevas. Source: USA TODAY Network; Reuters; Agriculture Department; Centers for Disease Control and Prevention.
